Absorption of laser energy by water is the basis of laser to tissue interaction of infrared lasers such as holmium (2080 nm to 2140 nm) and thulium (1900 nm to 2000 nm) FIG. 1. Within endosurgical techniques, surgical fields are also water based (e.g. saline or ringers). Laser energy is absorbed strongly within any gap between the laser fiber exit aperture and the tissue. In pulsed laser techniques, the first portion of each laser pulse is spent in boiling water, producing a steam bubble referred to as a “Moses bubble” in the field of art. Where pulse energies are low, e.g. 0.2 joules, and the fiber tip to target distance is considerable, e.g. 1 mm, most or all of the laser energy may be consumed in simply boiling water.
Prior art has addressed this issue for pulsed lasers by providing closely spaced or overlapping pulses: a small pulse to produce a vapor bubble though which a second, larger pulse may pass with minimal interaction with the surgical irrigant. As taught by U.S. Pat. No. 5,321,715, laser energy traveling in a liquid medium toward a target tissue will be absorbed, but that absorption may be less than expected due to the “Moses Effect”. As in the Biblical reference, the waters are parted by a first component of the pulse energy in producing a vapor bubble (Moses bubble) within the liquid medium. The remaining pulse energy passes through the far less attenuating medium of the bubble, resulting in higher that initially anticipated coupling of energy to the target.
The '715 patent describes a pulse format to increase the amount of laser energy which will arrive at the target tissue. According to the description, a first short and low energy initiation pulse is generated in order to create a bubble, followed by a higher energy treatment pulse. The second (treatment) pulse, when it passes through the created and now-formed bubble, experiences a lower absorption rate due to the presence of the bubble (and the absence of liquid). Moreover, the '715 patent teaches that the energy of the first bubble initiation pulse be sufficient enough to initiate the formation of a vapor bubble. The bubble thus formed may then displace a substantial portion of the fluid medium between a tip of a laser fiber and a target tissue.
Additional prior art has concentrated upon optimization of the Moses Effect. U.S. Pat. No. 5,632,739 teaches that a delay between a bubble initiation pulse and a treatment pulse is chosen so that the second pulse is emitted when the bubble size and corresponding amount of displaced fluid is at its maximum extent. U.S. Pat. No. 9,895,196 teaches optimization of reduced retropulsion (movement of target calculus away from the laser pulse source) in alternative timing of laser pulses.
Retropulsion is a is a phenomenon that is highly variable in real-world surgery and appears to be a function of laser pulse energy and repetition rate as well as fiber tip to target distance, stone composition and stone location in the anatomical region. In timing the second laser pulse for delivery just as the bubble begins to collapse, the '196 patent teaches the stone will be drawn in to the beam at the same time it is repulsed by the second pulse, maintaining the stone at a fixed distance from the fiber, where the separation of fiber and target is critical to the optimization of energy coupling efficiency.
U.S. Pat. No. 6,998,567 teaches the production of a multi-pulse train primarily for improved energy efficiency in generating the laser pulses, but with a mention of overlapping pulses for enhancing acoustic and thermal effects upon the target.